Patch 7.3.5 introduces a new scaling system similar to what we have in the Legion zones, so every mob will scale to your level now in older zones too. Fortunately, the scaling is capped at certain levels for each expansion, you won't be fighting level 110 mobs at Barrens.

So if you farm old materials with a level 110 character, this change shouldn't effect you since you will one shot everything anyway. However, this will make it harder to farm with lower level characters.

Skinning and Tailoring

Based on the tests I did on PTR, there was no change in cloth or leather loot tables. So, even if the mobs scale up to a higher level, they will still drop the same leather and cloth as before. The level 58-60 mobs in Barrens still dropped Light Leather and Linen Cloth.

Enchanting

Many of the old enchanting materials that were used to level enchanting between 1 and 300 were replaced with new materials as part of an effort to simplify things. This will make Enchanting a bit easier to level since 100-300 will basically use the same materials instead of using 5 different kind of dusts and essences.

Patch 7.3 adds new crafting materials for every professions, and if I remember correctly, this is the first time new base materials were added to the game this late and not just at the start of the expansion. You should expect that these will be really expensive at first.

I made farming guides for all of these new materials so check them out if you want to farm them. But keep in mind that most of these farming places will probably be pretty crowded, since everyone want these new materials

General changes

Crafting professions are receiving several new ilvl 885 crafted items that use new Argus materials.

NOTE: You can only upgrade the new items to 935, the old crafted item's ilvl still stays at 900.

[Primal Sargerite] is the new [Blood of Sargeras]. You can get it by Alchemy transmutes, Argus World Quests, and gathering professions on Argus. Later on you can also get it with the [Boon of the Lightbearer] enchant, but you will need Revered with the Army of the Light faction before you can buy it. This enchants allow you to sometimes loot [Light's Fortune] which can contain Primal Sargerite.

Unlocking new Argus recipes

To unlock the new recipes for Blacksmithing and Leatherworking, you have to complete the Argus story quests up to The Child of Light and Shadow. You can see the full quest chain at this link, but you will automatically get a quest called The Hand of Fate to start this quest chain as soon as you long in when 7.3 goes live. After you completed the quests, you will get a quest at the ship for your profession, which unlocks the recipes.

For Alchemy, you can get the quests at Krokul Hovel after you follow Velen and meat Chieftain Hatuul.

Jewelcrafting and Tailoring quest will unlock on week 2 (september 5) once you finish the The Path Forward quest at Mac'Aree.

Profession Changes

To make it easier to level professions to 800, most recipes will stay orange for 20 more points. This is a huge change! For example, the blacksmithing 780-800 part needed around 120 Demonsteel Bar and 60 Blood of Sargeras at minimum to craft 30 Waistguard. (you probably needed to craft 50 if you were unlucky). But now you will only need 20 Demonsteel Bar and 10 Blood of Sargeras to craft 5 of them.

The rank 2 recipes have the skill ranges that the rank 3 used to have, so if you are really unlucky with the rank 3 Alchemy recipes, now you can level Alchemy to 800 with just the rank 2. Or you can level inscription to 800 without waiting for the Darkmoon Faire.

It seems engineering helms are the only one that didn't change because the helms were already yellow up until 800, so engineering was already easier to level than the other professions. But now with this change you actually don't even have to make helms, because you can make the previous recipe up to 800.

Patch 7.2 Profession changes

The Obliterum Forge should now always be available to players, and requires no questline.

All item level 815 crafted end-game gear will now be crafted at item level 835.

You can now upgrade your items with [Obliterum] 10 times instead of 8, allowing a max item level of 865.

Increased drop rate of many 3-star recipes. Cooking recipes included.

Blacksmithing

Several old patterns and some new ones were added. These are low level recipes there were removed from the game, so it's mostly for those who want these for transmog. They come from quests or dungeon drops.

I recommend Zygor's Leveling Guide. The guide is in an small window on your main screen in-game. All the detail that you need to complete the quest is included. You’ll never have to switch back to a website, or even the in-game main map to see where you have to go or what’s next. The guide is available for both Horde and Alliance.